Coaxial coupling for traveling-wave tubes



Jan. 13, 1959 s. SENSIPER COAXIAL COUPLING FOR TRAVELING-WAVE TUBESFiled Sept. 2, 1955 ATM/EWEV COAXIAL COUPLING FOR TRAVELING-WAVE TUBESSamuel Sensiper, Los Angeles, Calif., assignor to Hughes AircraftCompany, Culver City, Calif., a corporation of Delaware ApplicationSeptember 2, 1955, Serial No. 532,323 4 Claims. Cl. 315-35) The subjectinvention relates to traveling-wave tubes in general, and moreparticularlyto an improved means for coupling microwave energy to theslow-wave structure in a traveling-wave tube.

Heretofore a rectangular waveguide has been used for coupling microwaveenergy to the type of slow-wave structure known as a contrawound helix.Such a system has the disadvantages of narrow bandwidth and of beinginflexible in adjustment or adaptability to different conditions ofoperation.

It is, therefore, an object of the present invention to provide abroadband system for coupling microwave energy onto a stub loadedcontrawound slow-wave struc ture.

It is anotherobject to provide such a coupling which is flexible andadaptable to either coaxial lines or waveguide lines.

It is a further object to provide such a coupling which mechanicallysimple and easy to fabricate.

It is still a further object to provide such a device which provides ameans for varying many of the parameters of coupling from the slow-wavestructure to the input and output transmission lines.

Briefly, objects are achieved in the following manner. The innerconductor of a coaxial line is coupled at one point or two points,directly to the input end of a contrawound slow- A wave structure. Thecoaxial line may then be matched easily to any desired transmission lineby methods well known in the art.

The novel features which are believed to be characteristic of theinvention, both as to its organization and method of operation, togetherwith further objects and advantages thereof, will be better understoodfrom the following description considered in connection with theaccompanying drawing in which an embodiment of the invention isillustrated by way of example. It is to be expressly understood,however, that the drawing is for the purpose of illustration anddescription only, and is not intended as a definition of the limits ofthe invention.

In the drawing:

Fig. 1 is .a View of a traveling-wave tube illustrating one embodimentof the present invention;

Fig. 2 is a cross-sectional view of another embodiment of the device ofFig. 1;

Fig. 3 is a view of a further embodiment of the present invention;

Fig. 4 is a cross-sectional view of still a further embodiment of thedevice of Fig. 3; and

Fig. 5 is a schematic view of a variation of the struc ture of Fig. 2.

Referring now to the drawing and specifically to Fig. l, traveling-wavetube is shown including a glass envelope 12 disposed between an electrongun 14 and a collector electrode 16. A typical contrawound slow-wavestructure 18 is shown disposed along the path of the electron beam fromgun 14 to collector 16. Slow-wave nited States Patent 0 in accordancewith the present invention, these structure 18 consists of a series ofspaced circular elements 15 interconnected by longitudinal elements orrods 17 extending from opposite points on circular elements 15 so thatthe rods 17 are disposed in a plane. Input coaxial line 20 is shown tohave its inner conductor 21 coupled through metal shield 22 to the pointon the input end of. structure 18 directly opposite the firstlongitudinal element of the structure. A microwave window 24 provides apressure seal for the evacuated volume within the tube. In like mannercoaxial conductor 26 is coupled to the output end of structure 18 suchthat the center conductor 27 is mechanically connected to the point onthe output end of the structure which is directly opposite the lastlongitudinal member of the structure. Again, a microwave window 28provides a pressure seal for the vacuum within traveling-wave tube 10.Solenoid 36] provides a longitudinally magnetic field for confining andfocusing the electron stream within structure 18.

Referring to Fig. 2, .there is shown in cross section another embodimentof the tube of Fig. 1 taken through the first circular member of theslow-wave structure of a similar tube. Input coaxial :line 28 is splitat junction 32 and energy traveling therealong is equally distributed toinner conductors 34 and 36 which pass through metal shield 23 and aredirectly coupled to structure 18 at the symmetrical, with respect topoint 39 which rep resents the first longitudinal member of structure18, points 40 and 42.

Referring to Fig. '3, there is shown a system in accordance with thepresent invention for coupling microwave energy onto structure 18 usinga second contrawound slow-wave structure 44 which may be similar tostructure 18 and which is disposed concentrically about structure 18 insuch a manner that at each point the inductive reactance is equal andopposite to the capacitive reactan-ce for electromagnetically couplingmicrowave energy between structure 44 and structure 18. Coaxialconductor 46 is arranged such that its outer conductor is coupled tometal shield 48 and its inner conductor 58 is coupled through shield 48to structure 44 are point opposite the first longitudinal member ofstructure 44.

Referring to Fig. 4, there is shown another embodiment of the device ofFig. 3 in a cross seetional view taken through the first circular memberof the contrawound slow-wave structure of a similar tube. Inputconductor 46 divides at junction 52 so that energy traveling alongcenter conductor is evenly divided between inner conductors 54 and 56which each pass through metal shield 49 and are coupled directly tosynmmetrical, with respect to point 55 which represents the firstlongitudinal member of structure 44, points 56 and 58.

Fig. 5 shows schematically a generalized version oi the structure shownin Fig. 2, in which the first circular member of structure 18is partedby an arbitrary gap 41 between points 40' and 42 opposite point 39 whichrepresents the juncture of the first circular member with the firstlongitudinal or stub member. Points 48 and 42 are asymmetricallydisposed with respect to point 39 and couple, respectively, innerconductors 34 and 3 to structure 18. Interposed in conductor 34' is avariable delay line 35; and in conductor 36' is variable delay line 37.

In operation of the example of the present invention, as shown in Fig.1, microwave energy is coupled symmetrically to contrawound slow-wavestructure 18. The resulting traveling wave is caused to grow or beamplified as it propagates along the structure; and the microwave energyis coupled out of the structure onto coaxial line 26 at the output endof traveling-wave tube 10 in the same manner.

It may be desirable to adjust or control many of the ascents propertiesof the coupling from the coaxial line to the contrawound slow-wavestructure by feeding the structure at different points along theperiphery of the first ring of the structure, but since the energy fedto the structure must be in phase when it reaches point 39, shown onFig. 2, the structure is fed at two points 40 and 42 which aresymmetrical with respect to point 39.

The structure of Fig. 3 represents an embodiment in which the R. F.energy is electromagnetically coupled to the contrawound slow-wavestructure 18 by use of a second slow-wave structure 44 which is disposedas shown about the input end of structure 18. This type of coupling isknown as through-vacuum or coupled helices coupling. The operation ofthis embodiment is similar to that of the embodiment shown in Fig. 1except that the input energy is used to launch a wave along structure 44which is long enough so that substantially all the energy of thetraveling wave of structure 44 is coupled to a traveling wave instructure 18. This traveling wave then is caused to grow or be amplifiedalong the length of structure 18.

In the embodiment of Fig. 4 energy is fed to the outer structure 44 at apoint different from the point opposite 55 and, as discussed inconnection with Fig. 2, it is fed symmetrically at points 56 and 58.

In the operation of the variation of the invent-ion as shown in Fig. 5,microwave energy is fed to junction 32 where it is divided and dispersedalong conductors 34 and 36 toward points 40' and 42' on the input end ofstructure 18. Point 40 and 42' may be asymmetrically disposed and themagnitude of delay of delay lines 35 and 37 adjusted to provide in-phasecurrents along the first longitudinal member of the structure. Furtheradjustment of the coupling parameters may be provided by changing theposition and length of gap 41.

There has thus been disclosed a system for coupling the coaxial line tothe slow-wave structure of a travelingwave tube which is very broadbandand which has great flexibility both mechanically and electrically sincemany techniques are well known in the art for coupling the coaxial lineherein described to any other type of microwave transmission line.Further, the structure disclosed is mechanically simple and easy tobuild. Also, the device of the present invention provides a heretoforeunknown means for varying coupling parameters and, in accordance withthe structures shown in Figs. 3 and 4, the present invention providesthrough-vacuum coupling for use with contrawound coupling slow-wavestructures.

What is claimed is:

1. A broadband traveling-wave tube comprising a contrawound slow-wavestructure having a first transverse circular member and a first linearlongitudinal member coupled thereto at a point on the periphery of saidfirst circular member; coaxial coupling means for coupling microwaveenergy from a transmission line to said slowwave structure comprising: acoaxial transmission line; a junction for said line; a first and secondpoint on the first circular member of the contrawound slow-wavestructure which are disposed symmetrically on the periphery of saidfirst circular member with respect to the point at which the firstlongitudinal member is attached thereto; a first and a second coaxialtransmission line, said first coaxial transmission line being coupledbetween said first point and said junction, said second transmissionline being coupled between said second point and said junction in amanner to feed said slow-Wave structure at a point not directly oppositesaid first longitudinal member while nevertheless providing means forfeeding said slow-wave 4 structure in a manner such that currents fromsaid first and second transmission lines in said first longitudinalmember are in phase.

2. The structure according to claim 1 in which there is a gap in theperiphery of said first circular member in that portion of said firstcircular member between said first and second points opposite said pointat which said first longitudinal member is attached to the periphery ofsaid first circular member.

3. In a broadband traveling-wave tube, an envelope, at contrawoundslow-wave structure within said envelope and having a first transversecircular member .and a first linear longitudinal member coupled theretoat a point on the periphery of said first circular member, whereby saidstructure is the electrical equivalent of a contrawound helix; andcoaxial coupling means for coupling microwave energy from a transmissionline to said slow-wave structure comprising a first coaxial transmissionline; a junction for said first line; a first and second point on saidfirst circular member of said contrawound slow-wave structure, saidpoints being disposed symmetrically on the periphery of said firstcircular member with respect to the point at which the firstlongitudinal member is attached thereto; a second and third coaxialtransmission line coupled to said first line and entering from the sideof said envelope, said second coaxial transmission line being coupledbetween said first point and said junction, said third transmission linebeing coupled between said second point and said junction to feed saidslow-wave structure at a point not directly opposite said firstlongitudinal member while providing means for feeding said slow-wavestructure in a manner such that currents from said second and thirdtransmission lines are in phase in said first longitudinal member.

4. A broadband traveling-wave tube comprising: an envelope; acontrawound slow-wave structure within said envelope having a firsttransverse circular member and a first linear longitudinal membercoupled thereto at a point on the periphery of said first circularmember whereby said structure is the electrical equivalent of a unifilarcontrawound helix; and coaxial coupling means for coupling microwaveenergy from .a transmission line to said slowwave structure comprising:a first coaxial transmission line; a junction point for said line; firstand second points on said first circular member of said contrawoundslowwave structure, said points being disposed on the periphery of saidfirst circular member and being separated thereon by a finite distance;second and third coaxial transmission lines coupled to said junctionpoint of said first line and entering from the side of said envelope;said second coaxial transmission line being coupled between said firstpoint and said junction point; said third transmission line beingcoupled between said second point and said junction point to feed saidslow-wave structure in a manner such that currents from said second andthird transmission lines combine in phase in said first longitudinalmember.

References Cited in the file of this patent Great Britain Mar. 12, 1952

